The present invention relates to multiple beam antenna (MBA) systems, such as are useful for communication satellites. Specifically, the present invention provides a microwave multiple beam antenna system that simultaneously achieves closely spaced beams (high crossover levels) and high aperture efficiency (low spillover loss) with a relatively simple beam forming network.
Conventional MBA designs, typically for communication satellites, place the feed horn cluster of the antenna at the focal point of an offset reflector collimator, as shown in FIG. 1. The feed horns are designed to be relatively small for close packaging in the cluster to give reasonably high crossover levels (i.e., closely spaced beams). A small feed horn, however, produces a broad radiation pattern for illuminating the offset reflector. This results in much of the energy not being intercepted by the reflector, and gives rise to high spillover loss. On the other hand, if the feed horns are designed for more directive beams to reduce the spillover loss, the feed horns become larger, yielding wider beam separation, and thus lower crossover levels. The result s "holes" in the pattern coverage.
FIG. 1 illustrates a conventional multiple beam antenna configuration. A beam forming network (BFN) 11 supplies signals to a feed horn cluster 13. which illuminates an offset paraboloid reflector 15. If the feed horns 19 are made relatively small for close packaging and reasonably high crossover levels 17 (as shown in FIG. 2), a significant portion of the beam misses the reflector, becoming spillover loss 21. Alternative feed horns that produce more directive beams to reduce the spillover loss, produce low beam crossover levels 23 in the beams reflected from the offset paraboloid reflector, as shown in FIG. 3.
A partial solution to the spillover loss problem is described by the inventor in Wokurka, A Feed Cluster Image Reduction System, Digest, IEEE AP-S Symposium, Blacksburg, Virginia, Jun. 1987, pages 199-202. In the system there described, an "imaging" lens is used to produce an optically reduced image of a large feed horn cluster. The reduced image of the feed horns is then used to illuminate the collimating reflector or dielectric lens. A field lens is placed between the imaging lens and the objective lens to efficiently refract the energy from each feed horn onto the objective lens, thereby maintaining low spillover loss for each beam at the objective lens.
Another system that has been suggested is to form overlapping feed horn subclusters with a more complex beam forming network. With this approach, energy to be radiated in a beam is divided in the BFN and applied to several adjacent horns. This approach increases the feed aperture size, and narrows the feed radiation pattern, to more efficiently illuminate the reflector. Adjacent beams are produced by overlapping these clustered feed horns. However, this approach complicates the feed network greatly, particularly for millimeter wave length signals and/or systems using a large number of beams. This approach also adds significantly to waveguide or transmission line losses. Such increased complexity and losses are particularly pronounced at higher millimeter wave frequencies, where they are least tolerable.
Another proposed solution to the spillover loss problem is to build several antennas, each of which produces widely spaced beams that are a portion of the total required. The beams from the separate antennas are then interlaced in space to create the full coverage complement. Clearly, this approach adds much unnecessary weight and volume to the antenna system by adding more antennas.